1. Field of the Invention
The present invention relates to a controller (control system) for an automatic transmission.
2. Description of Related Art
Conventionally, P (parking) range, R (reverse) range, N (neutral) range, D (drive) range, 2 (second) range and L (low) range are provided for in an automatic transmission and a driver selects a range by moving a shift lever to one of plural positions, arranged in an I shaped pattern and corresponding to these ranges. A gear change is automatically selected in accordance with a detected traveling condition of the vehicle.
In another prior art system the shift lever positions are arranged in an H shape, similar to a manual shift, to allow movement of the shift lever to a range position for travel in an automatic gear change mode or movement of the shift lever to a shift position for travel in a manual gear change mode (See Japanese Patent Laid-Open No. Hei 3-199758). This latter system allows a driver to freely shift to a selected gear stage and to obtain a good "shift feeling."
In the automatic transmission having only the I pattern of range positions, a gear change jump (e.g. 1-3 gear change, 1-4 gear change, etc.) is sometimes made, responsive to a traveling condition of the vehicle. Further, in the automatic transmission having the I pattern of range positions, as well as those having an H pattern of shift positions, a gear change jump is sometimes made by manipulation of the shift lever, for example while driving the vehicle in the manual gear change mode.
In such cases, although it is ideal to make the gear change directly to a targeted gear stage (gear speed), the thermal load on the frictional engagement elements engaged during the gear change becomes great when the gear change is made directly to the targeted gear stage, thus reducing the durability of the frictional engagement elements.
While it is possible to increase the number of friction members of the frictional engagement devices or to increase their frictional area, such increases not only enlarge the size and weight of the automatic transmission, but also lead to a dragging loss in the friction members. Thus, the thermal load on the frictional engagement elements is reduced by making a gear change to an intermediate gear stage, between the present gear stage and the targeted gear stage, when a gear change jump is to be made.
A gear change to the targeted gear stage is made only after firmly completing the gear change to the intermediate gear stage, by switching to a solenoid valve for the gear change to the targeted gear stage when the ratio of input rotary speed to output rotary speed reaches a preset value, in order to suppress shift shock (See Japanese Patent Laid-Open No. Hei 1-188750).
However, the frictional engagement elements for making the gear change to the intermediate gear stage and the frictional engagement elements for making the gear change to the targeted gear stage are engaged by supplying oil to the hydraulic servos and the hydraulic pressure is raised by an accumulator, so that it takes a considerable time to raise the hydraulic pressure to that whereby the frictional engagement elements can be engaged. Accordingly, a gear change through an intermediate gear stage not only prolongs the time required for making the gear change (hereinafter referred to as "the gear change time"), but also conveys an undesirable sensation to the driver.
Accordingly, it is an object of the present invention to solve the aforementioned problems of the prior art controllers for automatic transmissions by providing a controller which prolongs the useful life (durability) of the frictional engagement elements and shortens the gear change time, even when a gear change jump is made, and conveys no undesirable sensation to the driver.